1. Field of Invention
The invention relates to an image quality analysis system and method that can measure color-color registration to perform evaluation of printer and copier image quality automatically.
2. Description of Related Art
It is well known that customer satisfaction can be improved and maintenance costs reduced if problems with copiers and printers can be fixed before they become serious enough to warrant a service call by the customer. While current technology exists to enable printers and copiers to call for service automatically when sensors detect certain operating parameters outside of permissible ranges, there is not a very comprehensive manner of detecting incipient system failure or automatically diagnosing when problems with image quality reach a level where human observers perceive a reduction in quality. This is caused not only by the large number of operating parameters that would need to be tracked, but also because these parameters are strongly coupled to one another. That is, a given parameter at a certain value may or may not be a problem depending on the values of other parameters.
It is well known that color-color registration is critical for obtaining the optimal print quality from color printers and copiers. Currently used techniques for color-color measurements typically rely on absolute position measurements. Absolute position measurements require expensive, very high-accuracy equipment (e.g., motion stages). Such measurements are also easily affected by skew of the measured print relative to the measurement device.
As such, there are problems with existing color-color measurement systems.
There is a need for image output devices, such as printers and copiers, to better self-diagnose problems relating to image quality. Applicants have found that to comprehensively and reliably measure the system performance of a printer or copier, the image quality of the output must be measured.
One exemplary embodiment of the systems and methods of the invention overcomes such problems by developing powerful diagnosing tools within a digital printer or copier for self-diagnosis and evaluation of image quality. Image quality analysis can be performed to monitor many aspects of the printed output of the printing system. Of particular importance to overall image quality is color-color registration.
In this embodiment, the system provides: one or more digital test patterns stored in memory (or stored in hard copy form in the case of copiers) for providing one or more hard copy output test images; an input scanner that can scan the hard copy test image to form a digital raster image; and an image quality analysis module that receives information about the position of the digital raster image and produces test results relevant to determination of image quality , particularly color-color registration. The input scanner and image quality analysis module may form part of the image output device or may be stand-alone components used to test the device. Optionally, a communication module may be provided that is capable of contacting a service department or a more sophisticated diagnostic module if further analysis or service is necessary, depending on the outcome of the image quality analysis. Alternatively, information relating to color-color misregistration may be used by a corrective procedure within the image output device being tested to calibrate the device to correct for detected misregistration.
The systems and methods of the invention allow highly accurate measurements that are robust against skew At the same time the technique relies only on relative measurements, which can be performed sufficiently accurate with standard input scanners. The technique therefore also allows an alternative embodiment, where a special sensor (for example a small RGB CCD array) is placed in the paper path of the output device to detect color registration errors, and allow subsequent correction without the need to use a full-page scanner.
A special test pattern and measurement technique is used to allow highly accurate measurements of color-color registration in an image output device that prints in CMYK color (Cyan, Magenta, Yellow, Black). The method has been demonstrated to be accurate and robust using relatively low-resolution CCD-based scanners. The technique is unique in that it automatically factors out errors originating from skew between the detector and the subject of measurement Moreover, this invention determines the registration of each of C, M, and Y relative to K such that each of the C-K, M-K, and Y-K measurements are independent, and can obtain higher accuracy by averaging results over two or more lines.
In the case of C-K measurements, two reflectance profiles, using the red channel of the scanner, are taken across upper and lower parts of the image. From these profiles the centroids of the K and C lines are calculated. The offset between the centroid of upper(C) line #3 and lower(K) line #3 is a measure of the C-K misregistration, but also includes a contribution due to skew of the image. The offset between the centroid of upper(K) line #1 and lower(K) line#1 is a measure of the skew. By subtracting the two offsets, a skew-independent measure of the C-K misregistration is obtained. The M-K and Y-K registration errors are determined in similar fashion, using green and blue scanner channels respectively.
The system and methods of the invention have potential for being built in to color printers to allow measurement and automatic correction of color registration within the image path.